While it is common for airlines to conduct some form of psychological screening before pilots are hired, standards vary and there are no global guidelines, said Mr Michael Daniel, a retired air accident investigator with the United States Federal Aviation Administration.

"There are no psychological standards set by the International Civil Aviation Organisation, only medical," he said.

Dr Philip Scarpa Jr, president of the US-based Aerospace Medical Association, said: "Most airlines do not perform any periodic mental health assessments after an initial screening during the hiring process.

"In addition to screening, though, education of the pilots, their families and others in the aviation community on what to look for in mental wellness and how to report it are also important measures the airlines can take."

Dr Hsin Chen Chung, an aviation expert and the director of Nanyang Technological University's Air Traffic Management Research Institute, said: "Many airlines will say they have all the rules and screening in place, but what is crucial is the degree of implementation."

Singapore Airlines (SIA) pilots told The Straits Times that the carrier's pre-hiring assessment, which typically takes several hours, is "quite intense".

Former pilot and current head of the diploma in aviation management programme at Republic Polytechnic, Mr H. R. Mohandas, said: "It's all quite scientific and calibrated, designed to pick up personal traits and characteristics.

"Different scenarios and situations are presented to test your logic, intellect and other skills."